This invention relates to ultrasonic imagers and moe particularly to a method of changing the frequency spectrum of the system without changing the transducer or reconfiguring the system.
In ultrasonic examinations, given a specific allowed aperture, the lateral and axial resolution increase linearly (to first order) as a function of frequency. Consequently, a physician may wish to operate with the highest frequency transducers available. Body absorption of ultrasonic energy, however, will result in attenuation of the sound beam at a rate of approximately 1 dB/MHz/cm, the absorption increasing as a linear function of frequency. Furthermoe, various tissues in the body may become specular reflectors at certain frequencies. Thus, real time frequency changes can provide some tissue characterization information. It is consequently advantageous for the clinician conducting the ultrasound examination to be able to adjust the transducer frequency to achieve maximum lateral resolution while echoes from the desired maximum penetration remain just above the noise floor of the system; the examiner may alternatively choose to vary the frequency to enhance or reduce speckle. These adjustments should occur with the transducer or ultrasonic probe on the patient, in real time.
The illustrative embodiment of the ultrasonic imager is an electronic steered beam, single-sector scanner (also known as a phased array system) that has been extensively patented by the present assignee. The following patents and copending applications are among those relevant: U.S. Pat. Nos. 4,155,260; 4,180,790; 4,180,792; 4,182,173; 4,211,948; 4,211,949; 4,217,684; 4,217,909; Ser. No. 094,178 filed on Nov. 14, 1979, now U.S. Pat. No. 4,311,922 C. M. Puckette, "Variable Excitation Circuit"; and Ser. No. 112,852 filed on Jan. 17, 1980, now U.S. Pat. No. 4,277,978 C. M. Puckette, "Adaptive Input Circuit".